What is radon? Radon is a radioactive gas that comes from decay of uranium. It is found in varying amounts almost everywhere in the earth's crust, and trace amounts could be found in virtually every air sample taken anywhere on earth. In the natural environment radon dissipates harmlessly into the atmosphere. Thanks to its short radioactive half-life (3.8 days), it usually does not build up into dangerous concentrations. That requires a steady supply of radon gas seeping into an enclosed area.

What is the health threat from radon? Like other radioactive materials, exposure to radon can cause cancer. This has been proven in several studies of coal miners, who are exposed to elevated levels of radon working underground. Many medical researchers regard radon as the second leading cause of lung cancer in the U.S., after smoking. The most recent study, published in the Journal of the National Cancer Institute, says that radon causes an estimated 14,400 U.S. lung cancer deaths a year and may be responsible for up to 30% of lung cancers among non-smokers.

How does radon get into our homes? In the late 1980s came the first news of potential danger from radon gas seeping into homes. Like all gases, radon easily passes through porous materials, such as the dirt under a home's foundation and then through drain seals, basement cracks and other tiny openings.

Are all homes in danger? Not at all. Regions underlain with greater amounts of uranium-bearing rock are more at risk than others, for one thing. Mostly these are northern and western states, although elevated levels of radon have been found as far south as Georgia. Heaviest concentrations have been found in Winnipeg, Canada.

This doesn't mean that all homes in these regions are equally at risk. Two homes sitting side-by-side can record markedly different radon readings, depending on their construction characteristics, ventilation and other factors. Furthermore, highest concentrations of radon will be found in basements or ground floors for slab homes. The higher up in a home or building you go, the lower the radon exposure. Levels also are apt to be higher in the winter months when doors and windows are shut tight than during summer.

How much radon is too much? The EPA has established a "safe" threshold of 4 pico-curies per liter (pc/l) of air. Above that level, they recommend corrective action by home owners.

This is an extremely conservative standard. A pico-curie is one trillionth of a curie, the standard measure of radiation. Most studies suggest that radon gas can be a genuine cancer hazard at levels two to four times the EPA's suggested "action level." However, numerous studies have failed to demonstrate any danger at the 4 pc/l EPA tolerance.

This has been the source of some controversy. The EPA estimates the average radon level in American homes at 1.25 pc/l and that 6% - about one in every 16 U.S. homes - have radon readings in excess of 4 pc/l. The ultimate cost to bring radon down to tolerable levels in these homes has been estimated at some $50 billion. Congress and many state legislatures have gotten into the act with bills that mandate radon testing and mitigation as part of real estate transactions. Since there is no demonstrable danger at 4 pc/l, critics accuse the EPA of crying wolf and encouraging costly, useless remediation - all of which comes out of the homeowners' pockets.

How do I find out if my home has elevated radon levels, and what can I do about it? The good news is that it's relatively easy to test for radon. EPA approved radon test kits are sold by many hardware stores at prices ranging from $15-$25. Some state and local EPA offices offer free testing service for home owners, or will recommend EPA-approved firms to hire.

Remediation also is relatively simple. Often the first step is to plug the sump or floor drain connected to drain tiles - a prime radon gathering system. Check valves can seal these opening but still allow water to drain away.

Another step is to caulk and seal all cracks and openings in the basement floor and walls. if radon persists, it may be necessary to install a sub-slab suction system. This is a pipe running through the basement floor, connected to the drain tile system, with a fan sucking air to the outside.

Depending on how elaborate the work gets, the cost of bringing down radon levels in your home can range from a few hundred dollars to a few thousand. Call your local EPA office to get names of radon reduction contractors who are certified under the EPA's Radon Contractor Proficiency Program as having suitable technical knowledge. Or else call 1-800-55-RADON for any further information.